Immix leaks memeory due to stale line mark state left around

[tianleq]

In the current immix implementation, line mark state is cyclic and it is never cleared. We rely on the fact that line mark state value will not be the same between different GCs. However, only 7 bits is reserved for line mark state, which means after 128 GCs, the line mark state would wrap around, causing some stale line mark state incorrectly marking a free line as unavailable.

[tianleq]

The fix is straightforward, whenever a line is freed, clear the line mark state and bulk clear the line mark state within a block when the block becomes free

[tianleq]

Fixed in #1268